This paper presents an investigation of the flow and heat transfer characteristics of the water wall tube of an ultra-supercritical circulating fluidized bed (CFB) boiler for a better boiler design. The operating pressure ranges from 10.5 to 32 MPa. The six-headed inner rifled tube used in the test has a diameter and wall thickness of 35 mm and 5.67 mm, respectively. Results show that the vapor quality during heat-transfer deterioration (vapor quality > 0.8) increases with increased mass flux or decreased heat flux in the subcritical pressure regions. The effects of the pressure and heat flux on the inner-wall temperature in the supercritical pressure region are remarkable in the high-enthalpy region (enthalpy > 2400 kJ kg(-1)). The effects of the heat flux and pressure on frictional pressure drop are more dominant than mass flux in both the subcritical and supercritical pressure regions. The optimized tube is found to exhibit good performance under ultra-supercritical parameters.